The age of imperialism is ended," Sumner Welles, Franklin D. Roosevelt's under secretary of state, declared in 1942. Welles would have been shocked to learn that six decades later a number of American foreign policy thinkers would matter-of-factly describe the United States as an empire. "The fact of American empire is hardly debated these days," Thomas Donnelly, a neoconservative foreign policy analyst, wrote in Foreign Affairs in 2002. Donald Rumsfeld's Defense Department asked selected historians what lessons Americans could learn from empires of the past. Marxists, to be sure, had always described the United States as an empire, and for generations conservative isolationists have complained that the American republic gave way to an empire with the Spanish-American War, or the world wars, or the Cold War. The America-as-empire theme has now been taken up by two eminent scholars who do not belong to the neocon, radical, or isolationist traditions. But the popularity of an idea does not necessarily indicate that it is well-conceived.

In Day of Empire, Amy Chua, a professor at Yale Law School, writes that her book is a response to Samuel Huntington's claim in Who Are We? (2004) that immigration is weakening American society by dividing it between Spanish and English speakers. On the contrary, Chua asserts, diversity is strength: "For all their enormous differences, every single world hyperpower in history -- every society that could even arguably be described as having achieved global hegemony -- was, at least by the standards of its time, extraordinarily pluralistic and tolerant during its rise to preeminence. Indeed, in every case, tolerance was indispensable to the achievement of hegemony."

Chua's first book, World on Fire: How Exporting Free Market Democracy Breeds Ethnic Hatred and Global Instability (2002), is a brilliant and provocative study of economic conflicts between ethnic majorities and minorities. But her attempt in her second book to build a monocausal ethnic-diversitarian theory of how to succeed in world politics is flawed in its conception.

Chua lumps societies as different as the 21st-century United States, the 17th-century Dutch Republic, and the Persian, Roman, Chinese, Mongol, Mughal, Spanish, and Ottoman empires together in the elastic category of "hyperpowers." This broad definition elides key distinctions. Great powers in a system of multiple states are fundamentally different from universal empires whose boundaries are those of a civilization. Militaristic continental land powers also need to be distinguished from countries such as the Netherlands, Great Britain, and the United States that achieve financial and commercial hegemony and maritime supremacy without militarily dominating other great powers. The United States today has the world's primary reserve currency and what the political scientist Barry Posen calls military "command of the global commons" of sea, air, and space, but it lacks the power to dictate policies to France, Germany, or Japan, much less China, Russia, or India. In the same way, British and Dutch naval mastery and financial supremacy never translated into British or Dutch military hegemony in the European state system.

Besides blurring these critically important distinctions, Chua conflates two radically different conceptions of tolerance. "By tolerance, I don't mean political or cultural equality. Rather, as I will use the term, tolerance simply means letting very different kinds of people live, work, and prosper in your society -- even if only for instrumental or strategic reasons." By this standard, illiberal, autocratic empires that were indifferent to the religions and customs of the ethnic groups on which they preyed were as "tolerant" as modern liberal democracies that grant equal rights to citizens of different ancestries and beliefs.

Chua compiles a long list of allegedly tolerant "hyperpowers" as evidence that there is a "path the United States could take that would virtually ensure its decline: a turn to xenophobic, anti-immigration policies." But the comparison between imperial expansion and voluntary immigration doesn't work. Of course, vigorous empires became increasingly multiethnic as they expanded because they were subjugating and incorporating more tribes! Not a single one of the "tolerant" empires that Chua cites, including the racist British Empire, was a voluntary federation of sovereign nations, like the European Union, or a nation-state enlarged by voluntary immigration, like the United States.

The relevant modern history does not support Chua's attempt to correlate ethnic diversity with economic and military success. All modern nation-states, including the multiracial, immigrant-friendly U.S. and multiethnic India, are much less ethnically diverse than the multinational empires that formerly ruled them. Yet far more progress in civil rights, economic growth, and democracy has occurred in post-imperial nation-states than ever occurred in the former empires. The most generous welfare states in the world were created in the 20th century by the ethnically homogeneous Nordic democracies. Equally homogeneous East Asian nation-states provide most of the success stories in economic development since World War II. Most failed states in the world, including Iraq, Afghanistan, and arguably Pakistan, are multiethnic polities with borders arbitrarily created by European or Soviet colonial administrators.

Nor does it make any sense to compare the United States to polyglot, multinational agglomerations like the dynastic and national empires of the past. The United States is a multiracial nation-state with an overwhelming Anglophone cultural majority, where the diaspora cultures of voluntary immigrants usually fade away after a generation or two as a result of assimilation and intermarriage. Indeed, there may be a tragic trade-off between diversity and social solidarity. The recent work of political scientist Robert Putnam shows that such measures of civic health as mutual trust and political participation decline in ethnically diverse areas in the United States. The economists Edward Glaeser and Alberto Alesina have argued that around half the difference in welfare spending between the United States and Europe is attributable to the greater ethnic and racial diversity of the U.S. population. This is hardly news. In the late 19th century, Friedrich Engels attributed the weakness of socialism in the United States to ethnic divisions among European immigrants and native-born Americans. Both the New Deal and the civil-rights movement achieved their goals during the period of restricted immigration between the 1920s and the mid-1960s, when rapidly melting ethnic differences among whites and a shrinking foreign-born population forged a supermajority secure enough in its common identity to support the integration of black Americans.

Reasonable people can disagree about the desirable scale and composition of legal immigration to the United States. But not even proponents of more generous immigration policies are likely to take seriously Chua's argument that the U.S. should take in more immigrants because the Achaemenid Empire at the height of its power roped together a lot of conquered ethnic nations.

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In his book Among Empires, Charles S. Maier, professor of history at Harvard, questions the very enterprise of comparing the United States to empires of the past. "Does the United States have an empire?" and "Can the United States possibly be construed as being an empire?" he asks. He answers both questions with a qualified no: "Far-flung military bases are a prerequisite for imperial influence but do not themselves constitute an empire." As to "whether the United States is an empire at home," he says, "Not yet, at least."

If the contemporary United States is not an empire, what is it? "Critics of the term empire have suggested that the United States is instead a hegemonic power," Maier writes. "Hegemon is a Greek term that means preeminence and leadership." The United States "has been 'merely' hegemonic in Europe and Latin America" and "potentially imperial" only in the Caribbean and possibly Afghanistan and Iraq.

These careful distinctions are persuasive. To carry out his comparative project, however, Maier frequently ignores the very distinctions he has drawn. For example, he treats U.S. policy toward American Indians as imperialism comparable to overseas colonialism: "Modern imperial forces have also met defeat, often in overconfident expeditions in frontier regions -- Custer against the Sioux in 1876, the British against the Zulu in 1879, the Italians against the Ethiopians in 1885 and again in 1896, the Spanish against the Moroccans in 1921." But in most of the European settler states of the Americas, as well as in Australia and New Zealand, white or mestizo majorities displaced and confined indigenous populations. It seems odd to describe Canada and Mexico as "empires" for this reason.

Even more questionable is Maier's use of the concept of empire in connection with trade and investment. Maier describes the global trading system as America's "empire of production" that has evolved into an "empire of consumption": "Free trade was the pendant of monetary hegemony. The 1960s became for the United States the analogue of the Victorians' 'empire of free trade,' an era in which the technological superiority of the major economic power meant that its interests coincided with as broad an opening of international markets as possible." Maier's discussion of U.S. trade policy, like his analysis of U.S. security strategy, is insightful and deeply informed. But what is gained by using the term "empire" instead of less loaded phrases such as commercial or financial hegemony?

Maier's reference to "the Victorians' 'empire of free trade'" is an allusion to an influential 1953 article by Ronald Robinson and John Gallagher, "The Imperialism of Free Trade." Robinson and Gallagher argue that private British overseas investment, much of which was outside of the formal British Empire, should be treated as part of the empire as well. By this definition, ranches in Wyoming and factories in Pittsburgh financed by British investors were part of the British Empire. Maier's notion of an "empire of consumption," in which Chinese factories making goods for the U.S. market are part of an American "imperial" system, is vulnerable to the same objection.

Maier is a subtle and erudite historian. But his ambitious attempt at comparative politics suffers from what he himself calls "the difficulty of shoehorning the United States into the received models of imperial power." If hegemony in a pluralistic state system is imperialism, and an unequal alliance is imperialism, and voluntary trade is imperialism, and the treatment of indigenous minorities is imperialism, then everything is imperialism, and the concept is too multivocal to be useful.

This might be dismissed as a scholastic quibble over semantics, were it not for the implications for contemporary politics. When distinguished mainstream scholars like Chua and Maier, and not just neoconservative polemicists like Victor Davis Hanson and Max Boot, seek to compare the United States to ancient and modern colonial empires, the Zeitgeist has become truly ominous.

What happened to the story that American liberals and moderate conservatives alike used to tell about the role of the United States in the world? Mid-20th-century liberal internationalists like Franklin D. Roosevelt and Wendell Willkie shared the not altogether implausible view that the secession of the American colonies from the British Empire, the Monroe Doctrine, the Open Door Policy in Asia, the Fourteen Points, and the Atlantic Charter were successive stages in the erosion of imperialism and the establishment of a single global society of sovereign states based on popular self-determination and united by collective security organizations and voluntary trade and investment. Marxists and realists have always treated liberal internationalism as imperialism in disguise -- to the benefit, it turns out, of neoconservatives who can cite both leftists and realists in attempts to show that their neoimperialism is the natural culmination of America's foreign policy tradition, rather than the aberration that it actually is. If liberal internationalism is to be restored as the basis of U.S. foreign policy, then the first step must be to insist that liberal internationalism is not simply another kind of imperialism and that the United States is not simply another empire.